1. Field of the Invention
The present invention relates to a device and a method for controlling an automatic steering system of a vehicle and particularly relates to a device and a method for checking the ability to implement a predefined setpoint travel direction variable for a vehicle.
2. Description of the Related Art
Providing vehicles with an automatic vehicle control system is known. An automatic lane departure warning system or a parking assistance system is an example of such an automatic vehicle control system. To assist the driver in parking in a parking space, the parking assistance system ascertains an advantageous parking trajectory, taking into account the dimensions of the parking space and a position of the vehicle in relation to the parking space. For example, the parking trajectory is made up of a consecutive series of linear segments, circular segments, and clothoids. Depending on the dimensions and the starting positions, the circular segments may require various constant steering angle requests. There is even a high probability that the radii, and therefore the required steering angle, may vary greatly.
FIG. 1 shows a coordinate system for illustrating one example of a steering angle request ascertained by a parking assistance system, as a function of time (graph 2). The abscissa of the coordinate system is time axis t. The ordinate indicates setpoint steering wheel angles α0, which at appropriate times must be adjusted at the steering wheel or at the steering linkage. Setpoint steering wheel angle α0 corresponds to a steering wheel angle request.
At a point in time t0 the parking operation is started at a setpoint steering wheel angle α0 of 0°. At points in time t1 and t2, setpoint steering wheel angle α0 assumes other values when the steering wheel is to be greatly deflected. Thus, the planned route for the parking operation provides for two circular arcs on which setpoint steering wheel angle α0 is constant. The parking operation is terminated at point in time t3. If the specified advantageous steering angle request is maintained, it is assumed that at point in time t3 the vehicle is in an advantageous position in the parking space.
An ascertained advantageous parking trajectory, such as the variation over time of the steering angle request in FIG. 1, for example, may be communicated to the driver during the parking operation via steering instructions. Alternatively, the ascertained advantageous parking trajectory may be automatically implemented using an automatic steering wheel control system. The setpoint travel directions (setpoint steering angles, for example) of the advantageous parking trajectory are automatically set during the parking operation at given points in time which result from the ascertained advantageous parking trajectory. For example, the particular predefined setpoint steering wheel angle α0 is automatically set at points in time t0 through t3. This provides additional assistance to the driver in parking in the parking space.
A suitable automatic steering wheel control system may be implemented using an electronic steering system (electronic steering chassis (ESC)/electromechanical power-assisted steering (EPS)), for example. However, reliably maintaining the advantageous parking trajectory requires that the predefined setpoint travel directions be maintained relatively precisely. This places great demands on the electronic steering system.
In practice, however, it is often not possible to maintain specified setpoint steering wheel angles α0. FIG. 2 shows a coordinate system for illustrating an example of imprecisely maintaining predefined setpoint steering wheel angles α0 of FIG. 1. The abscissa and the ordinate of the coordinate system, the same as for FIG. 1 above, correspond to time axis t and specified setpoint steering wheel angles α0, respectively. The ordinate also indicates ascertained actual steering wheel angles α while a vehicle having an electronic steering system executes the predefined parking trajectory. A partial detail from FIG. 2 is illustrated in enlarged scale for better understanding.
Graph 4 indicates actual steering wheel angle α which is actually set at the steering wheel at a point in time t via the electrical steering system. It is apparent from a comparison of graphs 2 and 4 that setpoint steering wheel angle α0 specified by the parking assistance system is only imprecisely maintained by the electrical steering system. For example, actual steering wheel angle α, which is set for a setpoint steering wheel angle α0 of 300°, may only be 295°. If difference Δ of 5° between setpoint steering wheel angle α0 and actual steering wheel angle α is not corrected, this may result in significant deviations when the predefined parking trajectory is executed.
In addition, in particular for very tight parking spaces, parking trajectories result which have comparatively large steering angles. If it is no longer possible for the vehicle to implement these comparatively large steering angles, the parking trajectory traveled deviates significantly from the ascertained advantageous parking trajectory.
It is often possible that errors in setting the desired setpoint travel direction may no longer be correctable by an intervention by the driver. In many cases this requires premature termination of the parking operation. In addition, the departure of the vehicle from the ascertained advantageous parking trajectory may result in a collision, for example with an adjacently parked vehicle.
Published European patent application document EP 1 650 103 A2 describes a steering system for a vehicle in which a yaw rate is compared to a steering angle. The travel direction of the vehicle may then be corrected in such a way that the steering angle matches the yaw rate. However, the signal of the yaw rate must be highly filtered on account of the noise of the sensor. This results in a time delay. In addition, a reliable yaw rate is ascertainable only at comparatively high speeds. Furthermore, ascertaining the possible deviations between the yaw rate and the steering angle and then correcting the direction of travel requires a relatively large amount of time.